Previous approaches to the problem of preventing threaded fasteners from loosening during high vibrational loading incorporate high thread friction or an external locking device. High thread friction can be achieved by a plastic material that compresses when the fastener is screwed together. Alternately, a thread distortion technique can be used to force the threads to interfere with one another in response to an axial force causing a high frictional load to develop during installation. However, neither approach works well when the temperature environment changes from the installation temperature. As the temperature rises, high thread friction can cause an irreversible metal to metal lock or weld to occur. The plastic material approach similarly is limited in high temperature applications because plastic can deteriorate and deform in response to temperature increases. As the temperature decreases, frictional loading of the threads generated during installation may decrease which would allow the fastener to loosen when vibrated. Plastic materials harden and may become brittle. Failures occur when the plastic breaks or looses its frictional hold on the elements of the fastener. Both approaches wear easily and exhibit a limited life after several installation and removal cycles.
An external locking device approach requires the installation of a locking member and thus requires an unobstructed access to the fastener. Sometimes the locking device is a complicated mechanism and special tools and training for proper installation and removal may be required to properly install the device in order to be effective. This type of locking mechanism cannot be used in many otherwise desirable locations such as on a gas turbine engine where access is limited.
Accordingly, it is an object of this invention to provide a self-locking threaded fastener for a gas turbine engine that employs a temperature responsive locking member that prevents the threaded shank and a threaded mating element from disengaging from each other when exposed to vibration and temperature extremes.
It is a further object of the present invention to provide a wear resistant fastener which is capable of many installation and removal cycles.
It is a further object to provide a fastener which can be used in areas of limited access without special tools.
It is also desirable to provide a fastener which and can be retrofitted on existing gas turbine engines.